Due to the broadcast nature of wireless communication, wireless networks lack of physical boundaries. No physical connection to the external wireless communication is open to eavesdroppers. Addressing security at the physical layer of wireless communication has become necessary to issue a physical layer. Physical layer security technology is based security model of Shannon, which is the solution to the wireless communication boundaries, validity and reliability of theoretical models.
In 1975, tapping channel model was proposed by Wyner in Bell Labs in first time that considered reliable security technology and communication technology communication together. FIG. 1 illustration, Wyner pointed: Only the tapping channel is worse than legitimate channel conditions (i.e. legitimate traffic has basic advantages of the channel). By “security code” to achieve unconditional secret communications is feasible. Wyner's model suggests that “unconditional secret” communication systems through two-step “safety code” to achieve unconditional secret communications is feasible: The first step is to establish legal communication channel conditions which have the better advantage of wiretap channel model, the second step is “secure coding” to “expand” the advantages of legitimate recipient. But Wyner did not give an approach how eavesdropping channel can worse than legitimate channel. Without the implementation of wiretap channel model, not giving “safe coding” encoding method and decoding method, in a very long time Wyner and others' research results have not been given sufficient attention and further development.
Beamforming (beam-forming) is a combination of antenna technology and digital signal processing technology, which purpose for directional signal transmission or reception. A concept derived from the adaptive antenna. Signal processing at the receiving terminal can be synthesized by a weighted multi-element antenna array receives signals from various quarters to become an ideal signal. From the antenna pattern (pattern) perspective, this is equivalent to the formation of a predetermined point on the beam. For example, the original full-conversion reception pattern became a zero point of maximum lobe pattern. The same principal applies to the transmitting terminal The antenna array elements feed amplitude and phase adjustment, and then desired shape pattern can be formed.
When using beam-forming technology, the prerequisite is to use multi-antenna system. For example, multiple input multiple output (MIMO) can be not only multi-receive antenna, but also be used multiple transmit antennas. As a result of using multiple antenna groups, the receiver from a transmitting radio signals corresponding to the same spatial streams (spatial streams), propagates transmission paths in order to achieve the legitimate user of spatial streams to achieve better reception performance. Eavesdropper only receive noise signal in outer space stream, so as to achieve the legitimate communication parties establish the purpose of the advantages of channel, this is establishing the first step in the completion of Wyner model.